The present invention relates to seals used, for example, between rotating and stationary members. More specifically, the present invention is useful for maintaining a tight connective means, such as for example, a seal that prevents or minimizes the escape of substances such as gases between for example, a rotary tube kiln and a stationary hood and in embodiments that may prevent or minimize substances such as air from entering the kiln, or a similar apparatus.
Rotary tube kilns, also referred to as rotary tube furnaces, are commonly operated at elevated temperatures and with contained atmospheres for the treatment of materials being processed therein. In some uses, the leakage of materials, such as gas from the kiln is hazardous and may be detrimental to its operation and to the health and safety of individuals working with the kiln. For example, the processing of some materials requires a reducing atmosphere, such as hydrogen or other reducing gas or gaseous mixtures. The escape of such gases from the kiln or similar apparatus or device can adversely affect the environment and may create a hazard. Furthermore, the infiltration of air from the surrounding atmosphere into the rotary tube kiln may be detrimental to the process, or even hazardous for example, creating an explosive hazard. These and other disadvantages are avoided or minimized with the present invention.
It is known to employ graphite rings as seals at the ends of rotary tubes to prevent the leakage of gas. However, the thermal expansion of the rotary tube along the longitudinal axis of the tube at elevated temperatures may exert considerable horizontal axial pressure against the graphite ring seal, and other kiln components, causing motor drag and premature wear. Furthermore, repeated expansion and contraction associated with kiln temperature cycles can result in the development of gas leaks around the seal. The integrity of the seal, such as a graphite seal, is affected not only by the linear expansion and contraction of the rotary tube along the horizontal (longitudinal) axis, but also by eccentricities of rotational motion. These and other disadvantages are avoided or minimized with the present invention.
U.S. Pat. No. 6,042,370 issued to Vander Weide discloses a graphite rotary tube furnace for operation in controlled atmospheres at temperature of 1,500xc2x0 to 2,800xc2x0, and which tube comprises a generally horizontal rotatable graphite tube slidably supported on water-cooled split ring graphite bearings. An atmospheric seal is maintained with the aid of a flexible gas tight seal comprising graphite sealing rings slidably pressed against either or both sides of a drive plate with one or more flexible bellows to impart a positive sealing spring type force. The bellows and the drive plate are preferably comprised of stainless steel to withstand the conditions of operation of the furnace. This patent also discloses a gas inlet provided within graphite sealing rings for the transmission of an inert gas to maintain a positive pressure around the drive plate and the outside of the graphite tube in the region of the product entrance end of the furnace.
U.S. Pat. No. 5,551,870 issued to Gale discloses a rotating seal for a rotary drum seal including a rotating seal member welded about the drum near each of its ends, each sealing against a rotationally stationary seal member joined with the associated kiln end hood through a flexible gas tight bellows. The stationary seal rides on a pair of rollers each bearing upon a track, preventing its rotation. The bellows accommodates longitudinal expansion and contraction of the drum. Hanging weights are used with pulley wheels and flexible cables to urge the non-rotating members against the rotating members with unvarying force.
U.S. Pat. No. 5,406,579 issued to Vallomy discloses a dynamic seal for limiting the passage of a gas such as air through an opening, by providing a sealing chamber in advance of the entrance to the low pressure chamber having a large plenum with a variable speed exhaust blower in its upper portion, and a grate situated in its lower portion for access of air to the plenum. The blower speed is responsive to differential pressure measurements taken in the low pressure chamber and immediately adjacent the exterior of the low pressure chamber. Upstream of the plenum are one or more mechanical seals or flexible baffles which contact the top of the solid charge material and ride over it as the solid material passes beneath the mechanical seal.
Atmospheric seal systems for use between rotating and stationary members such as between a rotating tube kiln and a stationary hood, are disclosed in co-pending application entitled ATMOSPHERIC SEAL ASSEMBLY FOR A ROTATING VESSEL, filed of even date herewith, the disclosure of which is totally incorporated herein by reference
The disclosures of each of the above U.S. Patents are incorporated herein by reference in their entireties. In embodiments of the present invention the appropriate components and processes of the aforementioned patents may be selected for the seal arrangement and apparatus of the present invention.
It is an object of the present invention to provide a system for atmospheric sealing of a rotary tube furnace during operation.
It is a further object of the present invention to provide a seal for a rotary tube furnace that will compensate for the longitudinal and radial thermal expansion and contraction of the rotary tube as a result of temperature cycles, as well as for eccentricities of motion, while maintaining the integrity of the seal.
Additionally, it is an object of the present invention to prevent or minimize the unwanted entry or escape of substances, such as hazardous gases from device and apparatus, such as rotary tube kilns and the like.
Further, it is another object of the present invention to minimize or avoid excessive forces to be exerted on kiln seals which force can cause motor drag and premature wear of the seal and wherein there can be selected flexible, inclusive of fully flexible bellows which bellows have for example, a force of zero pounds per sq. inch when compressed, to maintain a permanent or substantially permanent seal. A related object of the present invention is to provide a multi-axis mechanical support system for flexible bellows and to provide an insulated shroud or protectant around a tube end directly under the bellows to prevent radiant heat generated from the bellows adversely affecting the life tie of the bellows.
The present invention provides a multi-axis rotary seal system for rotary tubes and in aspects thereof the present invention is directed to a seal arrangement comprising:
a structural ring;
at least two multi-axis supports attached to and supporting the structural ring which, in turn, holds, or is attached to hold a seal ring, such as a graphite ring, in sealing engagement with an end of a kiln tube;
a fully flexible bellows having first and second ends, the first end attached to the structural ring, the second end attached to a stationary hood; and
a biasing device applying force to the structural ring in a direction such that the structural ring biases the seal ring against the end of the kiln tube, thereby maintaining a substantially gas-tight seal between the seal ring and the end of the kiln tube during expansions and contractions of the kiln tube.
Typically, from two to about five multi-axis supports, preferably two, are positioned at approximately equal intervals around the circumference of the structural ring. Each multi-axis support comprises:
a bracket attached to the stationary hood;
a first shaft attached to the bracket, a longitudinal axis of the first shaft being substantially perpendicular to a longitudinal axis of a rotary tube furnace;
a second shaft having a longitudinal axis substantially parallel to the longitudinal axis of the rotary tube furnace and connected at one end to the first shaft through a bushing mounted around the first shaft, the bushing allowing limited rotation of the second shaft about the longitudinal axis of the first shaft;
a linear bearing at another end of the second shaft;
a spherical bearing mounted about the linear bearing and supporting the structural ring.
The linear bearing allows movement of the structural ring along the second shaft and the spherical bearing allows movement of the structural ring about an axis substantially perpendicular to the longitudinal axes of both the first and second shaft.
In a preferred embodiment of the seal arrangement of the present invention, the end of the kiln tube includes a flange suitable for slidable engagement with a seal ring such as a graphite ring.
In one embodiment of the present invention, the biasing device comprises at least two pneumatic cylinders, each attached at one end to the structural ring and at the other end to the stationary hood. Typically, two to twelve or more pneumatic cylinders may be employed depending on the circumference of the rotary tube kiln.
In another embodiment of the present invention, the biasing device comprises a spring-loaded cam-follower clamping device that provides a clamping pressure on the flange extending from the wall of a rotating vessel, such as a rotary tube kiln, and a stationary seal ring, such as a graphite ring, to provide a seal between the rotating vessel and the stationary sealing ring. The cam-follower clamping device comprises a cam-follower which, in use, is rotatably pressed against a side of the rotating flange opposite another side of the flange that slidably engages the seal ring; and an opposing spring which presses against a surface of the structural ring opposite another surface of the structural ring that is attached to the stationary seal ring, maintaining the rotating flange and stationary seal ring in slidable engagement and providing a vapor tight seal at this juncture of the sealing ring and flange.
In another embodiment of the invention, an insulating thermal shroud is positioned concentrically within the flexible bellows covering the end of the rotary tube and protecting the flexible bellows from heat.
The rotary tube kiln may be comprised of various materials depending, for example, on the temperatures and other conditions, to which it may be subjected and the nature of the particulate and gaseous materials with which it may be used. Typically, the rotary tube and flange, and structural ring, are comprised of a heat and chemically resistant material, such as stainless steel or high temperature alloy. Other examples of materials include plastic, aluminum, coated plastics, and various metals, such as carbon steel, and the like. The seal ring is preferably a graphite ring. However, depending on conditions of use, such as temperature, other low friction materials, such as polytetrafluoroethylene, or the like, may be used. The perpendicularity of the tube flange and the sealability of its surface with the graphite ring may be assured by machining the mating surface of the flange, that is, the surface that contacts the stationary graphite ring to form a seal. The graphite allows the tube to rotate with little drag and maintains a tight or substantially tight seal, such as a seal that will prevent gas escape from the kiln. The flange may be at the end of the rotary tube, but will typically be positioned a short distance back from the end. The end of the tube will then pass through the graphite ring, steel ring, and bellows, and enter the exit hood. As the kiln temperature increases, the rotating tube will expand in length and the tube flange will push against the graphite ring, compressing the bellows. The multi-axis support compensates for expansion and contraction of the rotating tube kiln, due to temperature changes, and eccentricities of motion of the rotating tube kiln while providing support for the structural ring and attached seal ring. As a result, the bellows does not have to provide support to the structural ring and seal ring and thus may be made of an extremely flexible material. The belllows employed in the present system may be a flexible bellows. In one preferred embodiment, the bellows may be preferably a fully flexible bellows made, for example, of flexible material, such as an elastomeric material like rubber or plastic, or even of thin metal. The material need not provide the bellows with enough strength to support itself. The thermal shroud, protecting the flexible bellows from heat, may be made of various insulating materials such as ceramic fibers or the like.
The multi-axis rotary seal system of the present invention may be installed at either or both ends of a rotary tube kiln.